This invention relates to new lacquer binders containing high molecular weight polyamide imides and special polyisocyanates containing blocked isocyanate groups, to their use in coating compositions for heat-resistant substrates and to a process for the production of wire enamels using coating compositions containing these binders.
It is a-ready known that high molecular weight polyamide imides can be used for the production of highly heat-resistant films and coatings, more especially for wire lacquering (cf for example DE-AS No. 1 256 418, FR-P No. 1,375,461, U.S. Pat. No. 3,541,038 or U.S. Pat. No. 3,625,911). However, these polyamide imides, which are generally obtained by reaction of trimellitic anhydride with diisocyanates, are attended by the disadvantage that they are only soluble in highly polar, generally very expensive solvents such as, for example, N-methylpyrrolidone, dimethylformamide or dimethylacetamide. Accordingly, there has been no shortage of attempts to modify the polyamide imides in such a way that they may also be processed in other solvents, for example in cresols. The polyamide imides may be modified in this way by the use of lactams, particularly .epsilon.-caprolactam, during their production. Polyamide imides such as these containing lactams in incorporated form are described, for example in DE-AS No. 1 795 752. Although the polyamide imides according to this prior publication show very good solubility in the phenolic solvents normally used in wire lacquering, they are attended by the disadvantage that the lacquer films formed during the wire lacquering process are inferior in their mechanical and thermal properties to corresponding lacquer films based on analogous, but "lactam-free" polyamide imides.
It has now surprisingly been found that this quality disadvantage of the polyamide imides according to DE-AS No. 1 795 752 may be almost completely eliminated if the polyamide imides are used in combination with certain organic compounds containing carbodiimide and/or uretone imine groups and also blocked isocyanate groups.